Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power, etc.). Examples of the multiple access system include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc.
In a typical wireless communication system, one carrier is considered in general even if a bandwidth is differently configured between an uplink and a downlink. In 3rd generation partnership project (3GPP) long term evolution (LTE), one carrier constitutes each of the uplink and the downlink on the basis of a single carrier, and the bandwidth of the uplink is symmetrical to the bandwidth of the downlink in general. However, except for some areas of the world, it is difficult to allocate frequencies of wide bandwidths. Therefore, as a technique for effectively using fragmented small bands, a spectrum aggregation technique is being developed to obtain the same effect as when a band of a logically wide bandwidth is used by physically aggregating a plurality of bands in a frequency domain. The spectrum aggregation includes a technique for supporting a system bandwidth of 100 mega Hertz (MHz) by using multiple carriers even if the 3GPP LTE supports a bandwidth of up to 20 MHz for example, and a technique for allocating an asymmetric bandwidth between the uplink and the downlink.
The 3GPP LTE is based on dynamic scheduling to transmit and receive downlink data and uplink data. For downlink data transmission, a base station (BS) first reports a downlink resource allocation (referred to as a downlink grant) to a user equipment (UE). The UE receives the downlink data by using a downlink resource indicated by the downlink resource allocation. To transmit the uplink date, the UE first transmits an uplink resource allocation request (referred to as a scheduling request) to the BS. Upon receiving the uplink resource allocation request, the BS reports an uplink resource allocation (referred to as an uplink grant) to the UE. The UE transmits the uplink data by using an uplink resource indicated by the uplink resource allocation.
Hybrid automatic repeat request (HARQ) is a scheme of increasing transmission efficiency by combining error correction and retransmission. The HARQ can be divided into synchronous HARQ and non-synchronous HARQ. In the synchronous HARQ, a transmitter and a receiver can know a retransmission time in advance. In the non-synchronous HARQ, the receiver cannot know a transmission time of the transmitter in advance.
How to perform HARQ is not introduced yet when dynamic scheduling is used in a multiple carrier system, i.e., a system using a plurality of uplink carriers and a plurality of downlink carriers.